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Moving alphaMask


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I've got this issue and I'm not able to find any answers.
I have 2 images - background and a soft edged circle for a mask. Background stays in same place, but the circle should be able to move.
As far as I understand I can send mask coordinates when using alphaMask(source, mask, sourceRect, maskRect) by sending maskRect settings, but if I create rectangle (var rect2 = new Phaser.Rectangle(200, 200, 300, 300);) before applying alphaMask, the mask doesn't work. How can I send maskRect options? Or I should approach this somehow differently?

Current code I have:

var bmd = game.make.bitmapData(2141, 800);
bmd.alphaMask('BG_Dark', 'Mask');
game.add.image(0, 0, bmd);

Now it simply places both the BG and Mask at 0,0. But I need the mask to be, for example, at 300,300;

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Ok, I actually managed to solve the first problem like this:

//Create custom bitmapData containing the mask
maskD = game.make.bitmapData(2141, 800);
maskD.draw('Mask', 200, 200, 200, 200);

//Apply the mask and add it to the game	
bmd = game.make.bitmapData(2141, 800);
bmd.alphaMask('BG_Light', maskD);
game.add.image(0, 0, bmd);


Now the other problem is redrawing it on update... What I am doing now on each update:

maskD.draw('Mask', sprite.x - 100, sprite.y - 100, 200, 200);
bmd.alphaMask('BG_Light', maskD);

It clears both the mask and the main image, and redraws it. But this is really laggy.
Can I somehow update masks position without constantly redrawing everything?

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